Recently, cellular type digital mobile communication systems demand that a lot of users be accommodated effectively using limited frequency resources. Particularly, attention is paid to spectrum spreading in which a lot of users share radio waves of a wide frequency band. Such a spectrum spread communication technique is disclosed in the following documents.
Non-patent Document 1: Andrew I. Viterbi, CDMA—Principle of Spread Spectrum Communications, Addison-Wesley Publishing Company, p. 29-31, 1995
Patent Document 1: Japanese Patent Application KOKAI Publication No. 2003-218334
Patent Document 2: Japanese Patent Application KOKAI Publication No. H06-164547
Patent Document 3: Japanese Patent Application KOKAI Publication No. H05-308344
Non-patent Document 1 proposes CDMA communication as a basic spectrum spread communication technique.
Patent Document 1 discloses a technique of reducing the high interference power mixed in the band of the desired signal to improve the desired signal-to-interference ratio after despreading, thereby reducing suppression of the reception sensitivity. The method of reducing suppression of the reception sensitivity in the document executes a correction process with a preset value for a frequency noise in an A/D (analog-to-digital) converted value before despreading at the time of demodulating a received signal subjected to spectrum spread in a mobile communication terminal device by despreading the signal after A/D conversion.
Patent Document 2 discloses a technique relating to a modulator and receiver for spectrum spread communication which converts a bit sequence of transmission data to a chip code sequence having a higher transfer rate than that of the bit sequence. The technique of the document uses frequency shift keying in secondary modulation at the time of performing spectrum spread with use of double stage modulation, and demodulates the primary modulation input signal with correlation detection on a frequency detection output on the receiver side.
Patent Document 3 discloses a technique relating to a spectrum spread communication device effective when a non-desired signal appears in a narrow band in the band. The communication device of the document performs A/D conversion of a received signal, performs Fourier transform on the acquired digital signal, performs a predetermined operation thereon to detect a non-desired signal in a narrow band, and determines whether the non-desired signal exceeds an allowable limit value of communication quality based on the detected value.
Such techniques however have hardly considered adaptive handling of the high or low reception intensity to enhance the reception sensitivity.